"It's kind of expected to have some new faces in this sport, but I still wouldn't rule out Roger and Rafa and I would not take them out of that context of the best players in the world," Djokovic said at the Shanghai Masters on Tuesday.

He said he had never used the term "Big Four", used to group himself, Federer, Nadal and Andy Murray, who have dominated the rankings and major tournaments for a decade.

Djokovic pointed to Stan Wawrinka, who has won three major titles in the past three seasons, and former US Open champions Marin Cilic and Juan Martin del Potro.

"And now you have the new wave of tennis players, a new generation is coming up," he said, singling out Australia's Nick Kyrgios and the 19-year-old Croatian, Borna Coric.

"Kyrgios and Coric, these kind of players that are young and motivated to show to the world that they can compete on the highest level," Djokovic said.

Twenty-somethings Milos Raonic, Kei Nishikori and Dominic Thiem are all in the top 10, while Kyrgios and Lucas Pouille are also on the march and are already in the top 20.

Federer, who is missing the end of the season with injury, has dropped to seventh in the latest rankings, while Nadal is down in fifth.